1. Field of the Invention
This invention relates generally to the field of high load equipment actuators and, more particularly, to a high thrust linear actuator employing a satellite roller screw as a manual operator for large diameter valves in oilfield, gas and process flow applications.
2. Description of the Related Art
High thrust operators for large valves traditionally use high friction acme screws to actuate the high pressure valve gate. Typical systems employ very low efficiency acme screws with some form of right angle bevel or worm gear arrangement for mechanical advantage. High torque caused by friction in the lead screw nut is ameliorated to some extent through the use of worm gear reducers. However, while reducing torque, these units increase the number of revolutions of the operator required to close the valve. As an example, an acme screw with a 0.25 inch lead and 45% efficiency at 100,000 lbs thrust requires 8,842 in-lbs of torque. A 60:1 worm gearbox reduces torque to a manageable 147 in-lbs; however, the hand wheel must be operated at 90 degrees to the axis of the valve and the field technician would be required to turn the wheel 2,400 revolutions on the operator to obtain a 10 inch travel on the actuator.
Improved efficiency over the lead screw has been accomplished through the use of recirculating ball screws. A dramatic improvement in mechanical efficiency to approximately 92% is achievable. However, the load capacity of the ball screw is dramatically lower than a lead screw, requiring a very large diameter screw. Additionally, the ball screw design cannot be inverted resulting in an increase in the overall length of the operator. Alternatively, the screw stem extends through the operating wheel for the valve, advancing into the technician's face as the wheel is turned.
It is therefore desirable to have an operator design which reduces the number of required revolutions of the hand wheel to actuate the valve while maintaining high efficiency to reduce the load to a manageable level.
It is further desirable to employ an inverted design to allow reduction in the length of the operator for packaging efficiency while avoiding a 90 degree offset for the hand wheel.